It is a well known practice in the field of spark ignited gasoline engines, particularly those for automotive use, to provide means for heating the intake air-fuel mixture with heat transferred from the engine exhaust gases. Systems of this sort are primarily used to provide heat for vaporizing the fuel during the engine warm up period when the intake manifold is cold, the amount of heat applied being reduced or cut off completely after the engine has been fully warmed up to normal temperatures.
Such systems generally utilize juxtaposed manifold arrangements or interrelated passages wherein exhaust gases are directed against a plate or wall to which the intake manifold mixture is exposed, preferably at a point below the carburetor throat or inlet passage. The heating of the plate or wall by exhaust gases vaporizes fuel droplets impinging on the wall or plate and heats the adjacent air-fuel mixture. To control systems of this type generally requires a valve mechanically actuated by a thermostatic device or time controlled mechanism of some sort, adding complexity to the systems.
To overcome some of the problems inherent in mechanical valve systems, it has been proposed to use two-phase thermosyphons, more recently known as heat pipes, in which a vaporizable fluid is utilized to transfer heat from the exhaust gases to a fuel vaporizing chamber. The proposed systems have, however, involved relatively complex arrangements for vaporizing the fuel in advance of mixing with the main air charge and appear not to satisfy the desired requirements of simplicity and practicality, while also meeting the needs of fast warm up and automatic temperature limitation required of such systems.